Synaptic Transmission

Question 1

What are the 2 types of synapses ?

Answer 1

Chemical synapses - involves the secretion of a neurotransmitter
Electrical synapse - current flows between neurones

Question 2

What are the advantages of electrical synapses ?

Answer 2

Very fast - used for quick reflexes

Question 3

What is convergence ?

Answer 3

When several neurones connect to only 1 neurone at the same time

Question 4

What is divergence ?

Answer 4

When 1 neurone connects to 2 or more different neurones at the same time through different synapses

Question 5

What is a synaptic bouton ?

Answer 5

An unmyelinated swelling at the end of the axon
It contains mitochondria and small vesicles (synaptic vesicles)

Question 6

What is the approximate size of a synaptic cleft ?

Answer 6

20nm

Question 7

How does a chemical synapse work ?

Answer 7

• an action potential reaches the presynaptic terminal causing it to become depolarised
• voltage gated calcium channels open and calcium flows into the presynaptic terminal
• this triggers the fusion of synaptic vesicles with the presynaptic membrane causing secretion of neurotransmitter into the synaptic cleft
• neurotransmitter then diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane
• if the neurotransmitter activates a ligand gated sodium ion channel then synaptic transmission is fast and short lived
• if the neurotransmitter activates a G protein coupled receptor then transmission is slower and lasts longer

Question 8

Summarise vesicle release and recycling pathways

Answer 8

• Synaptic vesicles are filled with neurotransmitter and stored in the cytoplasm
• Active zones are moved to release sites where they dock
• Priming makes vesicles ready to release
• After exocytosis, vesicle proteins are retrieved by endocytosis
• Synaptic vesicles are regenerated in the nerve terminal via endoscopes
• Recycling vesicles exchange with the reserve pool

Question 9

What are the 3 types of pools where vesicles can be ?

Answer 9

• readily releasable pool
• recycling pool
• reserve pool

Question 10

What are the 2 main types of neurotransmitter receptor ?

Answer 10

1) ionotropic - ligand gated ion channels (faster)
2) metabotropic - G protein coupled (slower)

Question 11

What is the main neurotransmitter for synapses in the brain ?

Answer 11

Glutamate

Question 12

Summarise excitatory synapses

Answer 12

Fast excitatory transmission occurs when neurotransmitter is released from the pre-synaptic bouton and is able to bind to and open non selective cation channels. This causes depolarisation of the post synaptic cell and shifts the membrane potential closer to the threshold.
If the post synaptic cell is a neurone this is called excitatory post synaptic potential (EPSP)

Question 13

Summarise inhibitory synapses

Answer 13

Fast inhibitory transmission occurs when a neurotransmitter is released from the presynaptic terminal and is able to activate chloride or potassium channels in the post - synaptic membrane. Opening of these channels causes the membrane to become hyperpolarised.
This negative shift in membrane potential is known as inhibitory post-synaptic potential (IPSP).

Question 14

What are the 2 types of ionotropic receptor ?

Answer 14

1) fast AMPA - mediates regular transmission
2) slower voltage dependent NMDA receptors - these require depolarisation for activation due to a voltage dependent magnesium block and they play a role in synaptic plasticity

Question 15

What is summation ?

Answer 15

IPSP and EPSP events are not all or nothing events but rather are graded with the intensity of the activation. Because of the charge accumulation on the membrane they can be superimposed on each other.

Question 16

What is temporal summation ?

Answer 16

If a synapse is activated repeatedly the resulting synaptic potentials are added up

Question 17

What is spatial summation ?

Answer 17

When 2 synapses on different parts of the cell are activated at the same time and so the resulting synaptic potentials are added up

Question 18

What are motorneurones and motornerves ?

Answer 18

Motorneurones - control activity of muscle fibres
Motornerves - transmit signals from CNS to skeletal muscles

Question 19

What is a motor unit ?

Answer 19

A motoneuron and its innervated muscle fibre

Question 20

What is a neuromuscular junction ?

Answer 20

The contact between the motor axon and the muscle fibre

Question 21

How does a neuromuscular junction operate ?

Answer 21

• an action potential arrives at the synaptic terminal and depolarises it
• voltage gated calcium channels open and calcium ions flow into the synaptic terminal down their electrochemical gradient
• this triggers the fusion of docked synaptic vesicles with the plasma membrane
• Acetylcholine within vesicles is released into synaptic cleft, and then diffuses across the cleft and binds to nicotinic ‘ionotropic’ receptors on post-synaptic membrane
• ligand (ACh)-gated non-selective cation channels open and cause a depolarisation of the muscle membrane in the endplate region
• when the endplate region reaches threshold the muscle membrane generates an action potential that propagates along the muscle fibre triggering contraction
• remaining Acetylcholine is then hydrolysed by acetylcholinesterase in the synaptic cleft

Question 22

Is a neuromuscular junction a chemical or electrical synapse ?

Answer 22

It is a chemical synapse

Question 23

What is myasthenia gravis ?

Answer 23

It is an autoimmune condition which causes muscle weakness
It can cause drooping of the eyelid
It occurs because acetylcholine receptors are blocked by antibodies so no neuromuscular junction can occur

Question 24

Explain the types of acetylcholine receptors

Answer 24

Skeletal muscle - nicotinic receptors which are stimulated by acetylcholine and nicotine and blocked by curare
Cardiac and smooth muscle - muscarinic receptors which are stimulated by acetylcholine and muscarine and blocked by atropine

Question 25

What is present at inhibitory synapses ?

Answer 25

• GABA A (binds chloride ions)
• GABA B (binds potassium ions)

Question 26

What is short term synaptic plasticity ?

Answer 26

The amplitude of an EPSP is not fixed but dependent on the interval between successive action potentials. If the interval is short then successive EPSPs can either increase or decrease in amplitude.
Increase = short term facilitation
Decrease = short term depression

Question 27

What is long term synaptic plasticity ?

Answer 27

It is believed to be the mechanism by which we remember things.
After an intense burst of activation an increase in EPSP amplitude may last many hours and this is known as long term potentiation.
Long term depression happens if 2 neurones don’t fire at the same time.